As a process to treat a slurry of high concentration organic wastewater such as wastewater, from food manufacturing, organic sludge from sewage processing, and excrement from animate beings, anaerobic digestion is known in which methane that is reusable as an energy source is produced. However, anaerobic digestion is time-consuming and, moreover, about 50% of solid matter in influent wastewater is barely solubilized and the yield of methane is as low as about 50%.
As a method to increase solubility of solid matter in organic wastewater and improve transformation into methane gas, Japanese Unexamined Patent Publication No. 206785/1997 discloses a process wherein digested sludge is drawn out from an anaerobic digestion tank, denatured with ozone or high-voltage pulse discharge and, then, returned to the anaerobic digestion tank. Japanese Unexamined Patent Publication No. 179285/2001 discloses another process in which microorganisms producing lytic enzyme are employed. In this process, digested sludge is drawn out from an anaerobic digestion tank, subjected to solid-liquid separation and, then, thickened. Thereafter, the thickened digester sludge is treated with microorganisms producing lytic enzyme, treated with ozone under alkaline conditions and, then, returned to the anaerobic digestion tank.
Also since it has recently been found that the amount of deposited phosphorus is not enough for future demand, recycle of phosphorus is considered most important. Since organic wastewater, especially organic sludge from wastewater treatment, has a higher concentration of phosphorus, an attempt has been made to recover phosphorus from organic sludge. For example, “Renovation of polluted environment utilizing bio-remediation technology; biotechnology for recycling phosphorus”, Journal of Society of Environmental Science, Japan, Vol. 12, No. 4, pp. 433-441 (1999), discloses a process wherein activated sludge from a wastewater treatment plant is treated with heat at 70 to 90° C. for a maximum of 120 minutes to elute out phosphorus in the sludge into liquid phase.
Sludge in the anaerobic digestion tank forms sludge flocs where microorganisms and/or organic polymer are highly congested. In the above conventional process, however, ozone treatment is directly applied to solid matter in digested sludge so ozone hardly permeates into the sludge floes and acts only on the surface of solids in the sludge. As a result, hardly soluble substances in the digested sludge are not sufficiently denatured, i.e. not sufficiently converted into easily soluble substances, and improvement in solubility of sludge solids and transformation into methane are not satisfactory.
In the above conventional process, it is possible to disperse the solid matter with ozone in order to treat the hardly soluble substances inside the solid matter with ozone. However, when both dispersion and denaturation are performed with ozone treatment alone, ozone consumption increases. In other words, a large amount of ozone is required in denaturing the hardly soluble substances, i.e. in achieving conversion to easily soluble substances, so the process becomes inefficient and running costs thereof rise.
In the process of Japanese Unexamined Patent Publication No. 179285/2001, meanwhile, sludge is treated with ozone under alkaline conditions in order to produce radicals that are much more active than ozone. Since ozone is easily and rapidly decomposed into radicals under alkaline conditions, such conditions are preferable for producing radicals but the effects of ozone itself are neither obtained nor utilized through the process. In addition, since radicals derived from ozone are extremely active, they quickly react with dissolved contents (i.e. organic substances already dissolved) in the sludge rather than react with solid matter in the sludge. Accordingly, radicals for solubilizing the solid matter in the sludge are wasted and solublization of the solid matter is hardly enhanced under alkaline conditions. If improvement in solubilizing the sludge is sought under such alkaline conditions, a great amount of ozone must be injected.
In the above conventional processes in which ozone is utilized, ozone treatment is only intended to improve solubility of the sludge so neither the effect of ozone treatment to elute out phosphorus in the sludge into the liquid phase, nor a process to recover phosphorus, nor an apparatus for recovering phosphorus, is suggested. In the above conventional process in which phosphorus is recovered from the sludge of wastewater, the sludge is treated with heat to elute phosphorus out into the liquid phase. However, this heat treatment is only for eluting phosphorus out and other effects of this heat treatment, such as improvement in recovery of energy source, is not suggested. Therefore, an efficient process or apparatus, in which energy source and phosphorus resource are simultaneously recovered, is not realized.
Accordingly, the object of the present invention is to provide a process and apparatus in which solubility of solids in organic wastewater, organic sludge or digested sludge is improved, transformation of organic substances into methane is enhanced, and sludge disposal is reduced.
Another object of the present invention is to provide a process and apparatus, in which energy and resource are simultaneously recovered, where phosphorus is eluted out and recovered from solids in organic wastewater, organic sludge or digested sludge while organic substances are transformed into methane.
That is, the object of the present invention is to provide a process and apparatus in which solubility of sludge is efficiently improved with less energy and lower costs, the rate of energy recovery is increased, the amount of disposed sludge is reduced, and phosphorus is eluted out for recovery as a resource.